Universität Stuttgart

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Open Access
CO2-induced drastic decharging of dielectric surfaces in aqueous suspensions
(2024) Vogel, Peter; Beyer, David; Holm, Christian; Palberg, Thomas
We study the influence of airborne CO2 on the charge state of carboxylate stabilized polymer latex particles suspended in aqueous electrolytes. We combine conductometric experiments interpreted in terms of Hessinger's conductivity model with Poisson-Boltzmann cell (PBC) model calculations with charge regulation boundary conditions. Without CO2, a minority of the weakly acidic surface groups are dissociated and only a fraction of the total number of counter-ions actually contribute to conductivity. The remaining counter-ions exchange freely with added other ions like Na+, K+ or Cs+. From the PBC-calculations we infer a corresponding pKa of 4.26 as well as a renormalized charge in reasonably good agreement with the number of freely mobile counter-ions. Equilibration of salt- and CO2-free suspensions against ambient air leads to a drastic de-charging, which exceeds by far the expected effects of to dissolved CO2 and its dissociation products. Further, no counter-ion-exchange is observed. To reproduce the experimental findings, we have to assume an effective pKa of 6.48. This direct influence of CO2 on the state of surface group dissociation explains our recent finding of a CO2-induced decrease of the ζ-potential and supports the suggestion of an additional charge regulation caused by molecular CO2. Given the importance of charged surfaces in contact with aqueous electrolytes, we anticipate that our observations bear substantial theoretical challenges and important implications for applications ranging from desalination to bio-membranes.
Open Access
Triggered dissolution of electrostatically crosslinked hydrogels from star‐shaped polyampholytic block copolymers
(2026) Grün, Jonas Julius; Beyer, David; Mons, Peter Johannes; Seitel, Sebastian; Fribiczer, Nora; Poudel, Purushottam; Könemann, Nicklas; Zank, Lynn Kendra Renate Jagna; Zylla, Paul Fabio; Košovan, Peter; Seiffert, Sebastian; Holm, Christian; Schacher, Felix Helmut
We explore the possible reversible formation of hydrogels through electrostatic interactions between four‐arm star‐shaped block copolymers, consisting of a polyethylene glycol (PEG) inner block and either an anionic polystyrene sulfonate [PEG27‐b‐PSS108]4 or a zwitterionic polybetaine [PEG27‐b‐PCBMAAm110]4 as outer block. The combination of both can induce attractive or repulsive electrostatic interactions depending on the solution pH value and ionic strength. The polymers were synthesized using controlled atom transfer radical polymerization (ATRP). The charge of [PEG27‐b‐PCBMAAm110]4 was further investigated by potentiometric titration and zeta potential measurements. Using oscillatory shear rheology, we demonstrated the required conditions for hydrogel formation. Stable hydrogel formation is observed within a wide pH range (6.8 -9.5), corresponding to the protonation states of the carboxylic acid groups that facilitate electrostatic interactions. We also showed how the hydrogel stability is influenced by parameters like block copolymer concentration and ionic strength. Coarse‐grained simulations provided molecular‐scale insights, revealing charge regulation effects and the energetic favorability of electrostatic complexation up to high pH values. Overall, our results demonstrated the key design principles, as the polyelectrolyte length, ionic strength, and charge regulation effects, for the formation of partially reversible hydrogels, triggered by changes in the solution pH. Furthermore, we showed that understanding the desired conditions for hydrogel formation requires a combination of experimental characterization with modeling approaches.
Open Access
A sequence-specific theory for charge-regulating IDPs
(2026) Beyer, David; Holm, Christian; Wang, Zhen-Gang
Intrinsically disordered proteins are a notable class of biological polymers whose physicochemical properties and biological functions are determined by an intricate interplay of chain connectivity, electrostatic interactions, the sequence of residues, and nonuniversal short-range interactions. An important phenomenon in these molecules is charge regulation, which arises from weakly acidic and basic residues, but is often neglected in theoretical descriptions. In this work, we use the Edwards-Singh variational method to derive an approximate theory that describes sequence effects in the charge regulation and chain conformation of intrinsically disordered proteins. The main result of our theory is a set of coupled algebraic equations yielding a renormalized Kuhn length and residue-specific mean-fields that determine the respective ionization states of the residues. We discuss limiting cases of these equations that underline the internal consistency of our theory and connect our results to earlier studies. To solve the full set of equations, we propose a simple numerical scheme. As test cases, we calculate the conformation and ionization state of a weak polyelectrolyte and of 30 sequence variants of the polypeptide (EK)25. For the weak polyelectrolyte, we show that the theory predicts phenomena such as the overall suppressed ionization due to electrostatic interactions and the enhanced ionization at the chain ends. In the case of the polypeptide (EK)25, we find strong effects of the sequence on ionization, with well-mixed sequences exhibiting a broad pH range where the polypeptide is net neutral, while blockier sequences exhibit a steeper ionization response. Moreover, we also observe pronounced sequence effects on the swelling behavior of the chains.

Universität Stuttgart

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